Universal demodulation and modulation for data communication in wireless power transfer

ABSTRACT

The present invention provides a universal demodulation circuit, a load modulation circuit and associated method, and an associated power transfer system, all suitable for use in wireless power transfer. A power receiver with signal strength detection is also provided. Modulation of the impedance of the demodulation circuit is determinable by detecting the amplitudes of a first and a second electrical parameter, thereby demodulating data communicated by modulation of the impedance of the demodulation circuit. The modulation circuit has a communication modulator to modulate the impedance of the modulation circuit, to a predetermined minimum modulation depth, thereby to communicate data.

FIELD OF THE INVENTION

The present invention relates to demodulation and modulation circuits,particularly those used in data communication in wireless powertransfer.

BACKGROUND OF THE INVENTION

Wireless inductive power transfer related technology employs near-fieldmagnetic inductive coupling between an energy transmitter coil and anenergy receiver coil to transfer energy through a high frequency(typically hundreds of kilo-Hertz or even mega-Hertz) magnetic field.The energy transmitter coil typically forms part of a transmitter andthe energy receiver coil typically forms part of a receiver.

One important aspect of wireless power transfer is the accompanying data(information) communication between transmitter and receiver. Suchinformation communication serves at least, but not limited to, one ofthe following functions:

a) localization of receivers on the surface of the transmitter (i.e.load and load-position detection);

b) compatibility checking of the receiver through an identificationprocess (i.e. load identification);

c) configuring the transmitter or receiver based on the transferredinformation;

d) establishing a power transfer contract (a “power transfer contract”represents the parameters that characterize the power transfer);

e) exchanging power transfer status or error messages; and

f) monitoring of battery conditions.

The information communication can be bi-directional (from transmitter toreceiver and from receiver to transmitter) or in a single direction. Itcan be implemented with existing communication methods, such as thoseused in RFID, NFC, Bluetooth, Wi-Fi, or others (Partovi, US2007/0182367).

However, one disadvantage of such methods is that some communication ICand circuits need to be added to the transmitter and/or receiver forcommunication purposes, which introduces extra components, complexityand cost. In some instances, extra coils for data transfer are alsorequired.

In most cases, data transfer from the receiver to the transmitter ismore important and sometimes mandatory. One relatively simple method toachieve this purpose is by using a method called load modulation, inwhich some additional load impedance is switched on and off duringcommunication so that the total load impedance is changed. For example,a receiver can include a resistor or a capacitor which is switched onand off for communication purposes. In particular, the changed loadimpedance influences some electrical characteristics of the transmitterso that the data can be detected and re-constructed.

Indeed, such load modulation methods have been widely used in RFIDsystems. Normally, however, the amplitude change of one electricparameter (like the voltage across the transmitter coil or the currentthrough the coil) of the transmitter is detected and used fordemodulation. This is called “amplitude modulation and demodulation”. Ithas been analyzed and shown that such amplitude demodulation is alwaysvalid because the load modulation is very “prominent” in RFID systemsdue to very little power being transferred.

However, in wireless power transfer, data transfer is accompanyingenergy transfer. Load modulation must be analyzed with the considerationof different loading conditions. In fact, there is a very wide range ofloads having many different power requirements. This is totallydifferent to RFID systems. Furthermore, when differences in coupling dueto the different possible relative positions of the transmitter andreceiver and the different distances between the transmitter andreceiver are taken into account, load modulation becomes even morecomplex.

Moreover, the receiver may use many different methods to achieve loadmodulation. Besides the examples described above which use a resistor ora capacitor, some parameter in the receiver passive network (such anetwork can be a resonant tank, a filter or other functional networkformed from passive components) or even the impedance of the receivercoil itself can be changed for the purpose of data transfer.

Thus, in developing a universal transmitter to work with differentreceivers using different load modulation methods, simple amplitudedemodulation at the transmitter is not appropriate since it is notalways valid. Therefore, there is a need for a universal demodulationmethod in order to develop a universal transmitter for standardizedwireless power transfer.

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

SUMMARY OF THE INVENTION

The present invention provides in one aspect a demodulation circuit fora wireless power transfer device, the demodulation circuit having afirst, a second, and a third electrical parameter wherein one of theelectrical parameters is equal to the vector sum of the other twoelectrical parameters, and modulation of the impedance of thedemodulation circuit results in corresponding modulation of one or bothof the amplitude and phase of the first electrical parameter, thecorresponding modulation being determinable by detecting the amplitudeof the first electrical parameter and the amplitude of the secondelectrical parameter, thereby demodulating data communicated bymodulation of the impedance of the demodulation circuit.

Preferably, the demodulation circuit is passive.

Preferably, the demodulation circuit includes a first power transfercoil for inductive coupling with a second power transfer coil in amodulation circuit, such that modulation of the impedance of themodulation circuit results in corresponding modulation of the impedanceof the demodulation circuit, thereby allowing data to be communicatedwirelessly from the modulation circuit to the demodulation circuit.

In one embodiment, the demodulation circuit includes an inductorconnected in series with the first power transfer coil and a capacitorconnected after the inductor in parallel with the first power transfercoil, such that the vector sum of the voltage across the inductor andthe voltage across the first power transfer coil is equal to the voltageacross the demodulation circuit, and wherein the first electricalparameter is the voltage across the first power transfer coil, thesecond electrical parameter is the voltage across the inductor, and thethird electrical parameter is the voltage across the demodulationcircuit. In a variation, the amplitude of the voltage across theinductor is detected by detecting the amplitude of the current flowingthrough the inductor.

In another embodiment, the demodulation circuit includes a capacitorconnected in series with the first power transfer coil, such that thevector sum of the voltage across the capacitor and the voltage acrossthe first power transfer coil is equal to the voltage across thedemodulation circuit, and wherein the first electrical parameter is thevoltage across the first power transfer coil, the second electricalparameter is the voltage across the capacitor, and the third electricalparameter is the voltage across the demodulation circuit. In avariation, the amplitude of the voltage across the capacitor is detectedby detecting the amplitude of the current flowing through the capacitor.

In a further embodiment, the demodulation circuit includes a capacitorconnected in parallel with the first power transfer coil, such that thevector sum of the current flowing through the capacitor and the currentflowing through the first power transfer coil is equal to the currententering the demodulation circuit, and wherein the first electricalparameter is the current flowing through the first power transfer coil,the second electrical parameter is the current entering the demodulationcircuit, and the third electrical parameter is the current flowingthrough the capacitor.

Preferably, the demodulation circuit includes a controller for detectingthe amplitudes of the first and the second electrical parameters.

In one embodiment, the controller is adapted to directly detect both theamplitudes of the first and the second electrical parameters.

In another embodiment, the demodulation circuit includes a signal bufferfor detecting one of the amplitudes of the first and the secondelectrical parameters and sending corresponding data to the controller,wherein the controller is adapted to directly detect the other of theamplitudes of the first and the second electrical parameters.Preferably, the signal buffer is a shift register or a secondcontroller. More preferably, the second controller is of lowerfunctionality or complexity when compared to the first controller.

In a further embodiment, the demodulation circuit includes a logicnetwork for detecting both the amplitudes of the first and the secondelectrical parameters and performing a logical “or” function on theamplitudes, wherein the controller is adapted to receive the results ofthe logical “or” function.

Preferably, the controller is a micro-controller-unit.

In another embodiment, the demodulation circuit performs one or more ofthe following functions: a resonant tank, impedance matching, andfiltering.

Preferably, the demodulation circuit forms part of a wireless powertransmitter, wherein the first power transfer coil transmits powerwirelessly to the second power transfer coil.

In another aspect, the present invention provides a modulation circuitfor a wireless power transfer device, the modulation circuit having acommunication modulator to modulate the impedance of the modulationcircuit thereby to communicate data, the communication modulatorselected to modulate the impedance to a predetermined minimum modulationdepth.

Preferably, the impedance has an active part and a reactive part, thevector sum of the active and reactive parts being equal to theimpedance, and wherein the communication modulator modulates one or bothof the active and reactive parts to modulate one or both of theamplitude and phase of the impedance.

Preferably, the modulation circuit includes a second power transfer coilfor inductive coupling with a first power transfer coil in ademodulation circuit, such that modulation of the impedance of themodulation circuit results in corresponding modulation of the impedanceof the demodulation circuit, thereby allowing data to be communicatedwirelessly from the modulation circuit to the demodulation circuit.

In one embodiment, the communication modulator includes a communicationresistor connected in parallel with a load in the modulation circuit,the communication resistor adapted to be switched on and off to modulatethe impedance of the modulation circuit.

In one variation, the modulation circuit includes a first capacitorconnected in series after a second capacitor, the communication resistorconnected between the first and second capacitors and in parallel withthe first capacitor. Preferably, the modulation circuit includes asecond power transfer coil connected in series before the secondcapacitor.

In another variation, the modulation circuit includes a capacitorconnected in series before the communication resistor and the load.Preferably, the modulation circuit includes a second power transfer coilconnected in series before the capacitor.

In a further embodiment, the communication modulator includes acommunication capacitor connected in parallel with a load in themodulation circuit, the communication capacitor adapted to be switchedon and off to modulate the impedance of the modulation circuit.Preferably, the modulation circuit includes a second capacitor connectedin series before the communication capacitor and the load. Preferably,the modulation circuit includes a second power transfer coil connectedin series before the second capacitor.

In one embodiment, the active and reactive parts are in quadrature andthe voltage across the modulation circuit can be assumed constant, suchthat the predetermined minimum modulation depth can be expressed as

$\frac{\sqrt{\left( {I_{sa}^{\prime} - I_{sa}} \right)^{2} + \left( {I_{sr}^{\prime} - I_{sr}} \right)^{2}}}{\sqrt{I_{sa}^{2} + I_{sr}^{2}}} \geq {r\; e\; q}$

wherein:

-   -   req is the predetermined minimum modulation depth;    -   I_(sa) is the active part of the total current flowing through        the modulation circuit before modulation;    -   I_(sr) is the reactive part of the total current flowing through        the modulation circuit before modulation;    -   I_(sa)′ is the active part of the total current flowing through        the modulation circuit after modulation; and    -   I_(sr)′ is the reactive part of the total current flowing        through the modulation circuit after modulation;

whereby the capacity of the communication modulator required to satisfythe predetermined minimum modulation depth can be calculated, and thecommunication modulator is selected based on the calculated capacity.

In another embodiment, the active and reactive parts are in quadratureand the total current entering the modulation circuit can be assumedconstant, such that the predetermined minimum modulation depth can beexpressed as

$\frac{\sqrt{\left( {V_{sa}^{\prime} - V_{sa}} \right)^{2} + \left( {V_{sr}^{\prime} - V_{sr}} \right)^{2}}}{\sqrt{V_{sa}^{2} + V_{sr}^{2}}} \geq {r\; e\; q}$

wherein:

-   -   req is the predetermined minimum modulation depth;    -   V_(sa) is the active part of the voltage across the modulation        circuit before modulation;    -   V_(sr) is the reactive part of the voltage across the modulation        circuit before modulation;    -   V_(sa)′ is the active part of the voltage across the modulation        circuit after modulation; and    -   V_(sr)′ is the reactive part of the voltage across the        modulation circuit after modulation;

whereby the capacity of the communication modulator required to satisfythe predetermined minimum modulation depth can be calculated, and thecommunication modulator is selected based on the calculated capacity.

Preferably, the modulation circuit is passive.

Preferably, the modulation circuit forms part of a wireless powerreceiver, wherein the second power transfer coil can receive powerwirelessly from the first power transfer coil.

In a further aspect, the present invention provides a power receiver forreceiving and transferring power to a load, the power receiver includinga rectification circuit connected before the load, and further includinga voltage detector for detecting the voltage before the rectificationcircuit.

Preferably, the power receiver includes an auxiliary rectifier connectedbefore the voltage detector.

Preferably, the power receiver includes the modulation circuit describedabove, wherein the data includes the detected voltage.

Preferably, the power receiver includes a second power transfer coil forinductive coupling with a first power transfer coil in a powertransmitter, thereby allowing wireless power transmission from the powertransmitter to the power receiver, and wherein the detected voltageindicates power signal strength between the power transmitter and thepower receiver.

In yet another aspect, the present invention provides a power transfersystem including the demodulation circuit described above and themodulation circuit described above.

Preferably, the power transfer system includes a wireless powertransmitter and a wireless power receiver, the wireless powertransmitter adapted to transmit power wirelessly to the wireless powerreceiver. Preferably, the wireless power receiver includes themodulation circuit and the wireless power transmitter includes thedemodulation circuit, thereby allowing data to be communicatedwirelessly from the wireless power receiver to the wireless powertransmitter. Preferably, the wireless power receiver is the powerreceiver described above.

In a further aspect, the present invention provides a method ofmodulating the impedance of a modulation circuit to communicate data,the method including:

determining the capacity of a communication modulator such that thecommunication modulator can modulate the impedance to a predeterminedminimum modulation depth;

providing the modulation circuit with the communication modulator, suchthat the impedance of the modulation circuit can be modulated with thecommunication modulator, thereby to communicate data.

Preferably, the impedance has an active part and a reactive part, thevector sum of the active and reactive parts being equal to theimpedance, and wherein determining the capacity of the communicationmodulator includes determining the vector sum of the active and reactiveparts such that the impedance can be modulated to a predeterminedminimum modulation depth by modulating one or both of the amplitude andphase of the impedance with the communication modulator.

Preferably, the method includes providing the modulation circuit with asecond power transfer coil for inductive coupling with a first powertransfer coil in a demodulation circuit, such that modulating theimpedance of the modulation circuit results in corresponding modulationof the impedance of the demodulation circuit, thereby allowing data tobe communicated wirelessly from the modulation circuit to thedemodulation circuit.

In one embodiment, the method includes providing the communicationmodulator with a communication resistor connected in parallel with aload in the modulation circuit, the communication resistor capable ofbeing switched on and off to modulate the impedance of the modulationcircuit.

In one variation, the method includes providing the modulation circuitwith a first capacitor connected in series after a second capacitor, thecommunication resistor connected between the first and second capacitorsand in parallel with the first capacitor. Preferably, the methodincludes providing the modulation circuit with a second power transfercoil connected in series before the second capacitor.

In another variation, the method includes providing the modulationcircuit with a capacitor connected in series before the communicationresistor and the load. Preferably, the method includes providing themodulation circuit with a second power transfer coil connected in seriesbefore the capacitor.

In a further embodiment, the method includes providing the communicationmodulator with a communication capacitor connected in parallel with aload in the modulation circuit, the communication capacitor capable ofbeing switched on and off to modulate the impedance of the modulationcircuit. Preferably, the method includes providing the modulationcircuit with a second capacitor connected in series before thecommunication capacitor and the load. Preferably, the method includesproviding the modulation circuit with a second power transfer coilconnected in series before the second capacitor.

In one embodiment, the active and reactive parts are in quadrature andthe voltage across the modulation circuit can be assumed constant, andthe method includes calculating the capacity of the communicationmodulator required to satisfy the predetermined minimum modulation depthby using the following expression:

$\frac{\sqrt{\left( {I_{sa}^{\prime} - I_{sa}} \right)^{2} + \left( {I_{sr}^{\prime} - I_{sr}} \right)^{2}}}{\sqrt{I_{sa}^{2} + I_{sr}^{2}}} \geq {r\; e\; q}$

wherein:

-   -   req is the predetermined minimum modulation depth;    -   I_(sa) is the active part of the total current flowing through        the modulation circuit before modulation;    -   I_(sr) is the reactive part of the total current flowing through        the modulation circuit before modulation;

I_(sa)′ is the active part of the total current flowing through themodulation circuit after modulation; and

I_(sr)′ is the reactive part of the total current flowing through themodulation circuit after modulation.

In another embodiment, the active and reactive parts are in quadratureand the total current entering the modulation circuit can be assumedconstant, and the method includes calculating the capacity of thecommunication modulator required to satisfy the predetermined minimummodulation depth by using the following expression:

$\frac{\sqrt{\left( {V_{sa}^{\prime} - V_{sa}} \right)^{2} + \left( {V_{sr}^{\prime} - V_{sr}} \right)^{2}}}{\sqrt{V_{sa}^{2} + V_{sr}^{2}}} \geq {r\; e\; q}$

wherein:

-   -   req is the predetermined minimum modulation depth;    -   V_(sa) is the active part of the voltage across the modulation        circuit before modulation;    -   V_(sr) is the reactive part of the voltage across the modulation        circuit before modulation;    -   V_(sa)′ is the active part of the voltage across the modulation        circuit after modulation; and    -   V_(sr)′ is the reactive part of the voltage across the        modulation circuit after modulation.

Preferably, the modulation circuit is passive.

Preferably, the method includes connecting the modulation circuit into awireless power receiver, wherein power can be wirelessly transferredfrom the first power transfer coil to the second power transfer coil.

BRIEF DESCRIPTION OF THE FIGURES

Preferred embodiments in accordance with the best mode of the presentinvention will now be described, by way of example only, with referenceto the accompanying figures, in which:

FIG. 1 is a block diagram of a wireless power transmitter in accordancewith an embodiment of the present invention;

FIG. 2 is a circuit diagram of a demodulation circuit in accordance withan embodiment of the present invention, the demodulation circuit formingpart of a wireless power transmitter;

FIG. 3 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 2, showing the results of amplitude modulation;

FIG. 4 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 2, showing the results of phase modulation;

FIG. 5 is a circuit diagram of a demodulation circuit in accordance withanother embodiment of the present invention, the demodulation circuitforming part of a wireless power transmitter;

FIG. 6 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 5, showing the results of amplitude modulation;

FIG. 7 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 5, showing the results of phase modulation;

FIG. 8 is a circuit diagram of a demodulation circuit in accordance witha further embodiment of the present invention, the demodulation circuitforming part of a wireless power transmitter;

FIG. 9 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 8, showing the results of amplitude modulation;

FIG. 10 is a vector diagram of electrical parameters of the demodulationcircuit of FIG. 8, showing the results of phase modulation;

FIG. 11 is a diagram of a controller of a demodulation circuit inaccordance with an embodiment of the present invention;

FIG. 12 is a diagram of a controller of a demodulation circuit inaccordance with another embodiment of the present invention;

FIG. 13 is a diagram of a controller of a demodulation circuit inaccordance with a further embodiment of the present invention;

FIG. 14 is a block diagram of a wireless power receiver in accordancewith an embodiment of the present invention, wherein the block includesa modulation circuit in accordance with an embodiment of the presentinvention;

FIG. 15 is a vector diagram of electrical parameters of the modulationcircuit of FIG. 14, showing the results of load modulation;

FIG. 16 is a circuit diagram of a modulation circuit in accordance withan embodiment of the present invention, the modulation circuit formingpart of a wireless power receiver and using a communication resistor forload modulation;

FIG. 17 is a circuit diagram of a modulation circuit in accordance withanother embodiment of the present invention, the modulation circuitforming part of a wireless power receiver and using a communicationcapacitor for load modulation;

FIG. 18 is a circuit diagram of a modulation circuit in accordance witha further embodiment of the present invention, the modulation circuitforming part of a wireless power receiver and using a communicationresistor for load modulation;

FIG. 19 is a vector diagram of electrical parameters of the modulationcircuit of FIG. 18, showing the results of load modulation, and assumingthat L_(s) and C_(s) resonate at the carrier frequency;

FIG. 20 is a circuit diagram of a modulation circuit in accordance withanother embodiment of the present invention, the modulation circuitforming part of a wireless power receiver and using a communicationcapacitor for load modulation;

FIG. 21 is a vector diagram of electrical parameters of the modulationcircuit of

FIG. 20, showing the results of load modulation, and assuming that L_(s)and C_(s) resonate at the carrier frequency;

FIG. 22 is a circuit diagram of a modulation circuit in accordance witha further embodiment of the present invention, the modulation circuitforming part of a wireless power receiver and using a communicationresistor for load modulation;

FIG. 23 is a vector diagram of electrical parameters of the modulationcircuit of FIG. 22, showing the results of load modulation, and assumingthat L_(s) and C_(s) do not resonate at the carrier frequency;

FIG. 24 is a circuit diagram of a prior art wireless power receiver,showing a rectification circuit and a voltage detector after therectification circuit;

FIG. 25 is a circuit diagram of a wireless power receiver in accordancewith an embodiment of the present invention, showing a rectificationcircuit and a voltage detector before the rectification circuit.

FIG. 26 is a circuit diagram of a wireless power receiver in accordancewith another embodiment of the present invention, showing arectification circuit, a voltage detector before the rectificationcircuit, and an auxiliary rectifier before the voltage detector;

FIG. 27 is a circuit diagram of a wireless power receiver in accordancewith a further embodiment of the present invention, showing arectification circuit and voltage detection before the rectificationcircuit; and

FIG. 28 is a graph of the performance of the circuit shown in FIG. 27.

DETAILED DESCRIPTION OF THE BEST MODE OF THE INVENTION

Referring to the figures, a demodulation circuit 1 for a wireless powertransfer device is provided. The demodulation circuit 1 has a firstelectrical parameter 2, a second electrical parameter 3, and a thirdelectrical parameter 4. One of the electrical parameters is equal to thevector sum of the other two electrical parameters. Furthermore,modulation of the impedance of the demodulation circuit 1 results incorresponding modulation of one or both of the amplitude and phase ofthe first electrical parameter 2. This corresponding modulation isdeterminable by detecting the amplitude of the first electricalparameter 2 and the amplitude of the second electrical parameter 3,thereby demodulating data communicated by modulation of the impedance ofthe demodulation circuit 1.

The demodulation circuit 1 is therefore a universal demodulationcircuit, and in general represents a universal demodulation technique,since the demodulation circuit can determine the initially appliedmodulation of the impedance of the demodulation circuit 1, viadetermining the corresponding modulation mentioned above, regardless ofwhether the modulation is of one or both of the amplitude and phase ofthe impedance.

The demodulation circuit 1 further includes a first power transfer coil5 for inductive coupling with a second power transfer coil 6 in amodulation circuit 7, such that modulation of the impedance of themodulation circuit results in corresponding modulation of the impedanceof the demodulation circuit. This thereby allows data to be communicatedwirelessly from the modulation circuit 7 to the demodulation circuit 1.

In the present embodiment, the demodulation circuit 1 forms part of awireless power transmitter 8, wherein the first power transfer coil 5transmits power wirelessly to the second power transfer coil 6. Thesecond power transfer coil 6 and the modulation circuit 7 form part of awireless power receiver 9. Thus, the first power transfer coil 5 canalso be referred to as the transmitter coil, and the second powertransfer coil 6 can also be referred to as the receiver coil.Embodiments of the modulation circuit 7 and receiver 9 are best shown inFIGS. 16, 17, 18, 20 and 22, which will be discussed in further detailbelow.

In other embodiments, however, the demodulation circuit 1 can form partof a wireless power receiver, wherein the second power transfer coil 6transmits power wirelessly to the first power transfer coil 5.

Returning to the present embodiment, as shown in FIG. 1, V_(IN) is theinput voltage to the demodulation circuit 1. It is normally the outputof a high frequency power inverter. The frequency of the power inverteris called the power signal frequency and is also equal to the carrierfrequency. Although V_(IN) can be changed for power flow control, duringthe short time of communication, it can be looked as a constant value.TX (abbreviation of “transmitter”) passive network is a circuit networkcomposed of passive components, which serves at least one of thefollowing functions:

1) resonant tank;

2) impedance matching; and

3) filtering.

Thus, the demodulation circuit 1 can be passive.

L_(p) represents the transmitter coil. In some embodiments thetransmitter coil is actually a plurality of transmitter coils. Thereceiver coil 6, which can be inductively coupled with the transmittercoil 5, is represented by L_(s) in the Figures.

FIG. 2 shows the expanded view of a particular variation of thedemodulation circuit 1 as it forms part of the transmitter 8. In thisvariation, the TX passive network is formed by inductor L_(t) andcapacitor C_(t). Together with L_(p), they form a LCL resonant tank.Z_(r) represents the reflected impedance from the receiver 9 when it iscoupled with the transmitter 8. Z_(r) is influenced by operatingfrequency, mutual inductance between both coils, impedance of thereceiver, including the receiver coil, an RX passive network, acommunication modulator and a load.

More particularly, the inductor L_(t) is connected in series with thefirst power transfer coil 5 (transmitter coil) and the capacitor C_(t)is connected after the inductor L_(t) in parallel with the transmittercoil 5, such that the vector sum of the voltage across the inductorL_(t) and the voltage across the transmitter coil 5 is equal to thevoltage across the demodulation circuit V_(IN).

The following is based on vector analysis. During the data communicationprocess, the impedance of the modulation circuit 7 of the receiver 9 ischanged due to the on-off switching of a communication resistor 10 or acommunication capacitor 11 in the modulation circuit 7. This is shown inthe embodiments depicted in FIGS. 16, 17, 18, 20 and 22, which will bediscussed in further detail below. Consequently, the complex vector ofthe reflected impedance Z_(r) changes accordingly, be it amplitudechange and/or phase change. Furthermore, the vector of the transmittercoil 5 voltage V_(p) changes with amplitude change and/or phase change.

FIG. 3 shows the vector diagram when V_(p) has amplitude change due toload impedance modulation. V_(p)′ is the vector of the transmitter coil5 voltage when load impedance has been changed during communication. Inthis case, by simply detecting the amplitude (envelop) of V_(p), thetransferred data can be extracted and reconstructed.

However, in some cases, load modulation only causes the vector of V_(p)to have phase change, as shown in FIG. 4, in which only detecting theamplitude (envelop) of V_(p) does not allow reconstruction of thetransferred data. It has been disclosed that both the amplitude andphase of an electrical parameter (like V_(p)) should be used fordemodulation. However, the phase shift detection normally requirescomplex digital signal processing and cannot be implemented with suchsimple envelop detectors.

In the present invention, however, an alternative approach is disclosed.As shown in FIG. 4, V_(IN) can be regarded as constant. V_(IN) is equalto the sum of V_(p) and V_(Lt) in vector sense. When V_(p) has onlyphase change, the voltage across L_(t) must have amplitude variation. Soby sensing the amplitude of both V_(p) and V_(Lt), the transferred datacan be detected and demodulated.

Thus, in the present variation of the present embodiment, the firstelectrical parameter 2 is the voltage V_(p) across the transmitter coil5 (the first power transfer coil), the second electrical parameter 3 isthe voltage V_(Lt) across the inductor L_(t), and the third electricalparameter 4 is the voltage V_(IN) across the demodulation circuit 1.

As V_(Lt) has a direct relationship with the current I flowing throughL_(t), one variation is to detect both V_(p) and I. Furthermore, underthe same principle, electrical parameters detected by such doublechannel amplitude demodulation are not limited to above examples. Doublechannel demodulation here generally refers to the amplitude demodulationof two variables (such as V_(p) and I or V_(p) and V_(Lt)).

The same principle can also be applied to other transmitter topologies.FIG. 5 shows another variation of the embodiment of the demodulationcircuit 1 where it forms part of the transmitter 8. In this variation,the TX passive network is formed by one capacitor C_(t) connected inseries with the transmitter coil 5. The vector sum of the voltage V_(ct)across the capacitor C_(t) and the voltage V_(p) across the transmittercoil 5 is equal to the voltage V_(IN) across the demodulation circuit 1.

In this variation, the first electrical parameter 2 is the voltage V_(p)across the transmitter coil 5 (the first power transfer coil), thesecond electrical parameter is the voltage V_(ct) across the capacitorC_(t), and the third electrical parameter is the voltage V_(IN) acrossthe demodulation circuit 1.

FIG. 6 shows a vector diagram when the amplitude of V_(p) is changed dueto load modulation, while FIG. 7 shows a vector diagram when the phaseof V_(p) is changed due to load modulation. For the latter case, theamplitude of V_(ct) also needs to be sensed. Alternatively, both V_(p)and I, being the current flowing through the capacitor C_(t), are sensedfor data demodulation.

In another variation of the embodiment of the demodulation circuit 1where it forms part of the transmitter 8, the TX passive network isformed by one capacitor C_(t) connected in parallel with the transmittercoil 5, as shown in FIG. 8. It must be noted that in this case, V_(p) isalways equal to the constant input voltage V_(IN) across thedemodulation circuit. Therefore, it cannot be used for demodulation.Instead, the current I_(p) flowing through the transmitter coil 5 isused. The vector sum of the current I_(Ct) flowing through the capacitorC_(t) and the current I_(p) flowing through the transmitter coil 5 isequal to the current I entering the demodulation circuit 1.

As seen from the vector diagrams in FIGS. 9 and 10, I_(p) can haveamplitude change (FIG. 9) or phase change (FIG. 10) due to loadmodulation. For the latter case (only phase change), the amplitude ofthe total current, I, (vector sum of I_(p) and I_(Ct)) also needs to bedetected. The key in this analysis is that I_(Ct) is not changed withload modulation as V_(p) (V_(IN)) is constant.

Thus, in this variation of the present embodiment, the first electricalparameter 2 is the current I_(p) flowing through the transmitter coil 5(the first power transfer coil), the second electrical parameter 3 isthe current I entering the demodulation circuit 1, and the thirdelectrical parameter 4 is the current I_(Ct) flowing through thecapacitor C_(t).

The demodulation circuit 1 further includes a controller 12 fordetecting the amplitudes of the first electrical parameter 2 and thesecond electrical parameter 3.

FIG. 11 shows one embodiment in which the controller 12 is amicro-controller-unit (MCU) with two interrupt, or other I/O, pins. Pin13 of the MCU is triggered by an incoming signal. As shown in FIG. 11,at the time of t1, the first electrical parameter 2, from thedemodulation circuit 1, triggers the pin 13. In an execution subprogram,the MCU 12 needs to calculate the time interval and/or determine thesignal type and/or save data and/or check data and/or finish othertasks. During the execution of the subprogram, the pin 14 may betriggered by the second electrical parameter 3 at the time of t2. TheMCU 12 can then choose at least, but not limited to, one of thefollowing options:

1) pause the subprogram and respond to the second electrical parameter3; and

2) stack the second electrical parameter 3 and finish the subprogramfirst.

Thus, in this embodiment, the controller 12 is adapted to directlydetect both the amplitudes of the first and the second electricalparameters 2 and 3.

Another embodiment of the detection portion of the demodulation circuit1 is shown in FIG. 12, in which one controller 12 and one signal buffer15 are used. As above, the controller can be a MCU. The first electricalparameter 2 is inputted directly into the MCU which takes control of allthe main functions, while the second electrical parameter 3 is inputtedinto the signal buffer 15 which only receives data and exchanges datawith the MCU 12. Thus, the signal buffer 15 detects one of theamplitudes of the first and the second electrical parameters 2 and 3 andsends corresponding data to the MCU 12, whereas the MCU is adapted todirectly detect the other of the amplitudes of the first and the secondelectrical parameters.

The signal buffer 15 can be implemented as a second MCU, a shiftregister, or other components with similar functions. If it isimplemented by a second MCU, and since much less functionality isrequired, this second MCU 15 can be a low-level one with lowerfunctionality, complexity and cost, compared to the first MCU 12, ormaster MCU. One advantage of this embodiment is that it can effectivelyavoid any risk of time interference, which can occur when two signals,such as both the first and second electrical parameters, are inputtedinto one MCU.

A further embodiment of the detection portion of demodulation circuit 1is shown in FIG. 13. Both the first 2 and the second 3 electricalparameters are inputted into a logic circuit 16 which implements an “or”function. The output of the logic circuit 16 can then be sent to the MCU12. This embodiment requires the time difference between receiving thefirst and the second electrical parameters (t2−t1) to be as small aspossible.

As described above, the reflected impedance Z_(t), as shown in FIGS. 2,5 and 8, is changed with load modulation of the modulation circuit 7.The change can be amplitude change and/or phase change. The reflectedimpedance is directly related to the impedance of the modulation circuit7. Therefore, it is desirable to provide a modulation circuit thatensures sufficient modulation depth. In doing so, a compatiblemodulation circuit can be developed to work with a standardizeddemodulation circuit, that is to say, a modulation circuit having a loadthat is compatible with a standardized demodulation circuit.

Accordingly, and referring to the figures, the modulation circuit 7 isin accordance with an embodiment of another aspect of the presentinvention. The modulation circuit 7 has a communication modulator 17 tomodulate the impedance of the modulation circuit thereby to communicatedata, the communication modulator selected to modulate the impedance toa predetermined minimum modulation depth.

In the present embodiment, the modulation circuit 7 includes the secondpower transfer coil 6 and forms part of the wireless power receiver 9.Thus, and as noted above, the first power transfer coil 5 transmitspower wirelessly to the second power transfer coil 6, and the secondpower transfer coil 6 is also known as the receiver coil. It will beappreciated, however, that in other embodiments, the modulation circuit7 can form part of a wireless power transmitter, wherein the secondpower transfer coil 6 transmits power wirelessly to the first powertransfer coil 5.

Returning to the present embodiment, FIG. 14 shows a block diagram ofthe wireless power receiver 9. For simplicity, except for the receivercoil L_(s), all the other receiver electronics (including the modulationcircuit 7, communication modulators 17, rectification circuits, and aload) are hidden in the box. V_(s) is the induced voltage across themodulation circuit 7 due to inductive coupling with the excitedtransmitter coil. If V_(s) is regarded as constant, I_(s) is changedduring the communication process due to load modulation. The change canbe amplitude change and/or phase change.

The impedance has an active part 18 and a reactive part 19, the vectorsum of the active and reactive parts being equal to the impedance. Thecommunication modulator 17 modulates one or both of the active andreactive parts to modulate one or both of the amplitude and phase of theimpedance.

In the embodiment shown in FIG. 15, I_(s) is divided into an active partI_(sa), which is in phase with V_(s), and a reactive part I_(sr), whichis in quadrature with V_(s). It will be appreciated that the product ofV_(s) and I_(sa) is mainly composed of the real power transfer to a load20 in the modulation circuit 7.

Therefore, the minimum modulation depth can be expressed as:

$\begin{matrix}{\frac{\sqrt{\left( {I_{sa}^{\prime} - I_{sa}} \right)^{2} + \left( {I_{sr}^{\prime} - I_{sr}} \right)^{2}}}{\sqrt{I_{sa}^{2} + I_{sr}^{2}}} \geq {r\; e\; q}} & (1)\end{matrix}$

where I_(sa)′ and I_(st)′ are the active and reactive parts, 18 and 19respectively, of the total current flowing through the modulationcircuit 7 after modulation of the impedance of the modulation circuit.req is a value representing the predetermined minimum modulation depth.The predetermined minimum modulation depth can be selected on the basisof any desired design requirements. It can also be incorporated intoagreed industry standards.

Alternatively, if I_(s) is looked as constant, the change of V_(s) canalso be used as a requirement of modulation depth, as expressed by (2):

$\begin{matrix}{\frac{\sqrt{\left( {V_{sa}^{\prime} - V_{sa}} \right)^{2} + \left( {V_{sr}^{\prime} - V_{sr}} \right)^{2}}}{\sqrt{V_{sa}^{2} + V_{sr}^{2}}} \geq {r\; e\; q}} & (2)\end{matrix}$

In this case, V_(s) is divided into an active part V_(sa), which is inphase with I_(s), and a reactive part V_(st), which is in quadraturewith I_(s). Analogously to the above, V_(sa)′ and V_(st)′ are the activeand reactive parts, 18 and 19 respectively, of the voltage across themodulation circuit 7 after modulation of the impedance of the modulationcircuit, and req is a value representing the predetermined minimummodulation depth.

FIG. 18 shows one variation of the present embodiment in which themodulation circuit 7 is part of the receiver 9. In this variation, thecommunication modulator 17 includes the communication resistor 10(R_(cm)) connected in parallel with the load 20. The communicationresistor 10 is adapted to be switched on and off to modulate theimpedance of the modulation circuit 7.

In this variation, the modulation circuit 7 also includes an RX passivenetwork, as indicated in FIG. 16, and is formed by a first capacitorC_(d) and a second capacitor C_(s). The first capacitor is connected inseries after the second capacitor, and the communication resistor 10 isconnected between the first and second capacitors and in parallel withthe first capacitor. The second power transfer coil 6 (receiver coil) isconnected in series before the second capacitor. A rectification circuitis also included, but is not shown in the FIG. 18 because R_(load) andR_(cm) can be reflected to be in front of the rectification circuitmathematically.

For straightforward analysis, it is assumed that L_(s) and C_(s) form aseries resonant tank at the carrier frequency, which as noted above isequal to the power signal frequency. Under this assumption, V_(s) isequal to V_(d). As shown in FIG. 19, the vector diagram becomes verysimple. From this vector diagram, it can be seen that I_(s) can bedivided into active part, I_(R), and reactive part, I_(cd). When thecommunication resistor 10 (R_(cm)) is switched on, I_(R) becomes largerand increases to I_(R)′ so that the vector of I_(s) is changed.Following the above principle, the value of R_(cm) can be calculated.

For example, if the minimum value of R_(load) is 5Ω (this is normallydetermined by the output voltage and maximum output power), to achieve aminimum modulation depth of 10%, the following expression can be used:

$\begin{matrix}{\frac{\sqrt{\left( {\frac{V_{d}}{{5\;\Omega}//R_{cm}} - \frac{V_{d}}{5\;\Omega}} \right)^{2} + 0}}{\sqrt{\left( \frac{V_{d}}{5\;\Omega} \right)^{2} + \left( {2\;\pi\; f\; C_{d}V_{d}} \right)^{2}}} \geq {10\%}} & (3)\end{matrix}$

Suppose that the carrier frequency f (equal to the power signalfrequency) is 110 kHz, and C_(d) has a capacity of 18 nF. Solving theabove equation yields the conclusion that R_(cm)≦50Ω. In other words,R_(cm) must be lower than 50Ω to achieve the required minimum modulationdepth. It will be further understood that when R_(load) has any valuehigher than 5Ω (so that output power is lower than maximum power), theupper limit of R_(cm) becomes larger than 50Ω. That is to say, acommunication resistor 10 equal to or lower than 50Ω is the universalrequirement to satisfy all loading conditions. Thus, this load dependentmodulation (calculated on the basis of the minimum of the load resistorR_(load)) is independent of changing loading conditions. It must also benoted that the above analysis can also apply to a variation of theembodiment in which the modulation circuit 7 does not include a parallelfirst capacitor, C_(d). In this case, as I_(cd) is always equal to zero,the analysis will become simpler.

The same principle can also be applied to the variation of the presentembodiment shown in FIG. 20 in which the communication modulator 17includes the communication capacitor 11 (C_(d)). The communicationcapacitor 11 is connected in parallel with the load 20, and is adaptedto be switched on and off to modulate the impedance of the modulationcircuit 7. A second capacitor C_(s) is connected in series before thecommunication capacitor 11 and the load 20, and the second powertransfer coil 6 (receiver coil) is connected in series before the secondcapacitor C_(s). The vector diagram of this variation is shown in FIG.21.

In this variation, I_(cd) is equal to zero without load modulation. Thefollowing expression applies:

$\begin{matrix}{\frac{\sqrt{0 + \left( {{2\;\pi\; f\; C_{d}V_{d}} - 0} \right)^{2}}}{\sqrt{\left( \frac{V_{d}}{5\;\Omega} \right)^{2} + 0}} \geq {10\%}} & (4)\end{matrix}$

Suppose again that the carrier frequency f (equal to the power signalfrequency) is 110 kHz. It can be solved that C_(d)≧29 nF. It will befurther understood that when R_(load) has any value higher than 5Ω (sothat output power is lower than maximum power), the lower limit of C_(d)becomes smaller than 29 nF. In other words, a communication capacitor 11equal to or higher than 29 nF is a universal requirement to satisfy allloading conditions. Again, this load dependent modulation (calculated onthe basis of the minimum of the load resistor R_(load)) is independentof changing loading conditions.

The two examples above both assume that L_(s) and C_(s) resonate at thecarrier frequency, so that V_(s) is equal to V_(d). However, in manycases, such a resonant condition cannot be met. In some cases, C_(s) isnot included in the modulation circuit 7 at all. For example, in thevariation shown in FIG. 22, V_(t) is the voltage across L_(s) and C_(s),both of which do not resonate at the carrier frequency. In this case,equation (2) is more suitable for an analysis.

As seen from the vector diagram shown in FIG. 23, V_(s) is composed ofthe active part 18, V_(d), and the reactive part 19, V_(r). When thecommunication resistor 10 is connected into the modulation circuit 7,the vector of V_(d) is shortened to V_(d)′, and the vector of V_(s) ischanged to V_(s)′. The following expression (5) is derived from (2):

$\begin{matrix}{\frac{\sqrt{\left( {{I_{s} \cdot \left( {R_{load}//R_{cm}} \right)} - {I_{s} \cdot R_{load}}} \right)^{2} + 0}}{\sqrt{\left( {I_{s} \cdot R_{load}} \right)^{2} + \left( {{2\;\pi\; f\; L_{s}I_{s}} - \frac{I_{s}}{2\;\pi\; f\; C_{s}}} \right)^{2}}} \geq {r\; e\; q}} & (5)\end{matrix}$

If, in the present variation, R_(load) _(—) _(min)=5Ω, L_(s)=30.3 μH,C_(s)=+∞ (meaning that there is no resonant tank in the modulationcircuit 7), f=110 kHz, and req=10%, then expression (5) can be solved todetermine that R_(cm) should be equal to or higher than 6.6Ω. This wouldbe a general requirement for all loading conditions.

A further variation of the present embodiment, the communicationmodulator 17 includes both a communication resistor and a communicationcapacitor, such as the communication resistor 10 and the communicationcapacitor 11 as described in the variations above. This provides furtheroptions for modulating one or both of the amplitude and phase of theimpedance of the modulation circuit 7 to communicate data.

The above principle can apply to any communication modulator 17 used inany load modulation technique, assuming the minimum modulation depthrequirement can be met under all loading conditions. This solves theproblems in ensuring a minimum modulation depth, allowing more freedomin designing modulation circuits. It will be appreciated that althoughexpressions (1) and (2) give explicit examples of defining minimummodulation depth, any method using vector analysis to define themodulation depth is covered by the present invention. It will also benoted form the above that the modulation circuit 7 can be passive.

Demodulation and modulation circuits such as those described above canbe used to transfer data from a wireless power receiver to a wirelesspower transmitter. Typical data to be transferred includesidentification, output voltage, output current, output power andtemperature.

Another example of transferred data is called “signal strength”, whichrepresents the strength of the power signal, which is in turn defined asthe oscillating magnetic field enclosed by a transmitter coil and areceiver coil. This data assists a transmitter to determine the positionof a receiver before power transfer starts (i.e. load positiondetection)

For example, if a transmitter consists of many windings, the windingscan be excited one by one or one group by one group. The sensed signalstrength on the receiver side needs to be transferred to thetransmitter. The winding(s) with the highest signal strength can beselected to be excited for power transfer. This process is named“selection”. As selection needs to be finished before the power transfercycle, the signal strength is normally the first message to be sent whenthe receiver's MCU is powered up. If there is no data transfer from thetransmitter to the receiver to inform it to send a specific message, thereceiver needs to send the signal strength message every time it ispowered up. It must be noted that the receiver's MCU is normally poweredby the output of the rectification circuit or by the charging deviceitself.

One prior way to detect the signal strength is by detecting the outputvoltage of the rectification circuit, as shown in FIG. 24, in which theload is not shown for simplicity and a capacitor at the rectifier outputis highlighted. The detected voltage of the rectifier output is inputtedinto a Communication and Control Unit (normally a MCU) in the receiver.The MCU controls the communication switch of the communication modulatorbased on the encoded data. With the demodulation and modulationdescribed earlier, for example, the data can be received andreconstructed by the transmitter.

One disadvantage of the abovementioned signal strength detecting methodis that the capacitor at the output of the rectification circuit isnormally very large in terms of capacitance (in order to reduce theoutput ripple). This results in a very slow decrease in voltage when thepower signal is removed. During the selection process, the load isnormally disconnected because it needs to wait for the transmitterconfiguration. Such a slow decrease in voltage is very disadvantageousto the selection process since it causes unnecessary delay for the nextwinding or winding group to be excited during the selection process. Ifthe transmitter has many winding cells to be scanned, the totalselection time becomes long and user-unfriendly.

One straightforward solution to this problem is to normally close thecommunication switch during the selection process so that the largecapacitor C, can be discharged quickly. However, this solution cannotwork:

1) when the communication modulator is implemented with the use of acapacitor (as shown in FIG. 17) instead of resistor; and

2) when the receiver controller is powered by the charging device itself(e.g. the communication and control functions can be a part of thecontroller of the device itself to save cost).

Another better and universal solution disclosed in this invention is todetect the voltage in front of the rectification circuit. As there is nolarge energy storage component in front of the rectification circuit,such voltage can decrease very quickly when the power signal is removed.So this detected voltage can be used to trigger the MCU to reset.Furthermore, this method is independent of the power source of the MCU.

An embodiment of this aspect of the invention is shown in FIG. 25. Inparticular, there is provided a power receiver 21 for receiving andtransferring power to a load. The power receiver 21 includes arectification circuit 22 connected before the load, and further includesa voltage detector 23 for detecting the voltage before the rectificationcircuit.

There can be two extensions based on this method:

1) the voltage can also be sensed before the RX passive network; and

2) an auxiliary rectifier 24 can be used.

As shown in FIG. 26, the auxiliary rectifier 24 is connected before thevoltage detector 23.

FIG. 27 shows one detailed embodiment of a circuit for the powerreceiver 21. C13 and C14 are very small resonant capacitors and form theRX passive network. C12 is the large capacitor at the output of therectification circuit 22. R10 represents the MCU of the power receiveror some other component(s) that consume very low power. R8, R9 and C15form the circuit network of the voltage detector 23 to detect thevoltage in front of the rectification circuit 22. In one variation, itis a potential divider with a low pass filter.

FIG. 28 shows the performance of the circuit of FIG. 27. Power transferends at 2 ms. V_(dc) (output voltage of rectification circuit 22, seeFIG. 27) decreases very slowly due to the very light load. However,V_(ac) (voltage in front of the rectification circuit 22) decreases toalmost 0 in only 1 ms.

In this embodiment, after removing the power signal of one winding orwinding group, the transmitter only needs to wait for at least 1 msbefore it can excite the next winding or winding group.

In another embodiment, the power receiver 21 includes the modulationcircuit 7 described above for communicating the detected voltage or“signal strength” data. In a further embodiment, the power receiver 21is the wireless power receiver 9, and in particular, includes the secondpower transfer coil 6 for inductive coupling with the first powertransfer coil 5 in the power transmitter 8, thereby allowing wirelesspower transmission from the power transmitter 8 to the power receiver 9.The detected voltage indicates power signal strength between the powertransmitter 8 and the power receiver 9.

In another aspect of the invention, a power transfer system is provided.One embodiment of this aspect is a power transfer system including thedemodulation circuit 1 and the modulation circuit 7, both as describedabove. In another embodiment, the system includes the wireless powertransmitter 8 and the wireless power receiver 9. As described above, thewireless power transmitter 8 is adapted to transmit power wirelessly tothe wireless power receiver 9. The wireless power receiver 9 includesthe modulation circuit 7 and the wireless power transmitter 8 includesthe demodulation circuit 1, thereby allowing data to be communicatedwirelessly from the wireless power receiver to the wireless powertransmitter. In a further embodiment, the wireless power receiver 9 isalso in accordance with the power receiver 21 described above.

In a further aspect, the present invention also provides a method ofmodulating the impedance of a modulation circuit to communicate data.One embodiment includes determining the capacity of the communicationmodulator 17 such that the communication modulator can modulate theimpedance to a predetermined minimum modulation depth, as described indetail above. The embodiment further includes providing the modulationcircuit with the communication modulator 17, such that the impedance ofthe modulation circuit can be modulated with the communicationmodulator, thereby to communicate data. In another embodiment, themodulation circuit is the modulation circuit 7, with the method beingcarried out as described in detail above.

As shown in the foregoing, the present invention provides a demodulationtechnique, at least in the form of the demodulation circuit described,that can demodulate any impedance change, be it a change in amplitude,phase or any combination of both. Thus, a universal demodulation circuitand technique is provided by the invention. Furthermore, thedemodulation circuit and technique provided does not require complex orcostly components and can be implemented with only passive components.

Notwithstanding the universal nature of the demodulation provided, thepresent invention also provides a method and corresponding modulationcircuit that ensures a minimum modulation depth. This provides aguideline for designing devices with modulation circuits that assist inguaranteeing maximum compatibility between modulating and demodulatingdevices.

The demodulation and modulation circuits and techniques provided by theinvention are especially suited to power transfer devices, particularlywireless power transfer devices. The circuits and techniques of theinvention for demodulation and load modulation are capable of handlingthe vast variety of loading conditions that occur in these types ofpower transfer applications. They are also capable of handling the othervariables involved in these applications, such as differences incoupling due to the different possible relative positions of thetransmitter and receiver and the different distances between them.

The present invention also provides a technique for improving thedetection of signal strength in power receivers, especially wirelesspower receivers, as well as a power receiver embodying this technique.Detected signal strength is one of the types of data that iscommunicated between coupled power receivers and power transmitters,using demodulation and modulation circuits such as those also providedby the present invention and described in the examples above.

Furthermore, the present invention provides a power transfer system thatincludes the demodulation circuit and associated techniques, and themodulation circuit and associated techniques provided by the invention.In particular, the power transfer system can be a wireless powertransfer system and can include the power receiver also provided by theinvention. It will be appreciated that the system can provide all of theadvantages discussed above exhibited by the various other aspects of thepresent invention.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention can be embodied in many other forms. It will also beappreciated by those skilled in the art that the features of the variousexamples described can be combined in other combinations.

1. A demodulation circuit for a wireless power transfer device, thedemodulation circuit having a first, a second, and a third electricalparameter wherein one of the electrical parameters is equal to thevector sum of the other two electrical parameters, and modulation of theimpedance of the demodulation circuit results in correspondingmodulation of one or both of the amplitude and phase of the firstelectrical parameter, the corresponding modulation being determinable bydetecting the amplitude of the first electrical parameter and theamplitude of the second electrical parameter, thereby demodulating datacommunicated by modulation of the impedance of the demodulation circuit.2. A demodulation circuit according to claim 1 wherein the demodulationcircuit is passive.
 3. A demodulation circuit according to claim 1including a first power transfer coil for inductive coupling with asecond power transfer coil in a modulation circuit, such that modulationof the impedance of the modulation circuit results in correspondingmodulation of the impedance of the demodulation circuit, therebyallowing data to be communicated wirelessly from the modulation circuitto the demodulation circuit.
 4. A demodulation circuit according toclaim 3 including an inductor connected in series with the first powertransfer coil and a capacitor connected after the inductor in parallelwith the first power transfer coil, such that the vector sum of thevoltage across the inductor and the voltage across the first powertransfer coil is equal to the voltage across the demodulation circuit,and wherein the first electrical parameter is the voltage across thefirst power transfer coil, the second electrical parameter is thevoltage across the inductor, and the third electrical parameter is thevoltage across the demodulation circuit.
 5. A demodulation circuitaccording to claim 4 wherein the amplitude of the voltage across theinductor is detected by detecting the amplitude of the current flowingthrough the inductor.
 6. A demodulation circuit according to claim 3including a capacitor connected in series with the first power transfercoil, such that the vector sum of the voltage across the capacitor andthe voltage across the first power transfer coil is equal to the voltageacross the demodulation circuit, and wherein the first electricalparameter is the voltage across the first power transfer coil, thesecond electrical parameter is the voltage across the capacitor, and thethird electrical parameter is the voltage across the demodulationcircuit.
 7. A demodulation circuit according to claim 6 wherein theamplitude of the voltage across the capacitor is detected by detectingthe amplitude of the current flowing through the capacitor.
 8. Ademodulation circuit according to claim 3 including a capacitorconnected in parallel with the first power transfer coil, such that thevector sum of the current flowing through the capacitor and the currentflowing through the first power transfer coil is equal to the currententering the demodulation circuit, and wherein the first electricalparameter is the current flowing through the first power transfer coil,the second electrical parameter is the current entering the demodulationcircuit, and the third electrical parameter is the current flowingthrough the capacitor.
 9. A demodulation circuit according to claim 3forming part of a wireless power transmitter, wherein the first powertransfer coil transmits power wirelessly to the second power transfercoil.
 10. A demodulation circuit according to claim 1 including acontroller for detecting the amplitudes of the first and the secondelectrical parameters.
 11. A demodulation circuit according to claim 10wherein the controller is adapted to directly detect both the amplitudesof the first and the second electrical parameters.
 12. A demodulationcircuit according to claim 10 including a signal buffer for detectingone of the amplitudes of the first and the second electrical parametersand sending corresponding data to the controller, wherein the controlleris adapted to directly detect the other of the amplitudes of the firstand the second electrical parameters.
 13. A demodulation circuitaccording to claim 12 wherein the signal buffer is a shift register or asecond controller.
 14. A demodulation circuit according to claim 13wherein the second controller is of lower functionality or complexitywhen compared to the first controller.
 15. A demodulation circuitaccording to claim 10 including a logic network for detecting both theamplitudes of the first and the second electrical parameters andperforming a logical “or” function on the amplitudes, wherein thecontroller is adapted to receive the results of the logical “or”function.
 16. A demodulation circuit according to claim 10 wherein thecontroller is a micro-controller-unit.
 17. A demodulation circuitaccording to claim 1 wherein the demodulation circuit performs one ormore of the following functions: a resonant tank, impedance matching,and filtering.